Specialist chemical and biological warfare units from the British Army are investigating the graves of Sergei Skripal's wife and son after being deployed to Salisbury in England.

The investigation into last week's nerve agent attack on the former Russian double agent and his daughter, Yulia, intensified this weekend, as UK military personnel moved in. British counter-terror police, who are leading the probe, said they requested help to remove "a number of vehicles and objects from the scene in Salisbury town centre as they have the necessary capability and expertise".

Targets included Skripal's car, an ambulance, air-ambulance helicopter and police vehicle, which were covered by people wearing protective gear and driven away on military lorries. Officers in hazmat suits were seen placing a blue forensic tent over a memorial stone for the former spy's son, Alexander, and nearby grave of his wife, Liudmila.

They were both thought to have died of natural illnesses but the cause of their deaths - in 2017 and 2012 respectively - have come under renewed focus following the brazen attempt on Skripal's life.

Specialist operations continued as Skripal and his daughter remained in a "very serious" condition in intensive care, while a police officer also exposed to the nerve agent is conscious and talking from his hospital bed.

Detective Sergeant Nick Bailey was among the first officers sent to the bench where the pair were found unconscious last Sunday afternoon, and then onwards to Skripal's suburban home.

Investigators are looking at the possibility that Skripal was targeted at his home, where sources said a package containing the nerve agent may have been delivered and opened in the presence of his daughter. It was thought significant that DS Bailey had visited the property.

Forensic tests have also been carried out at a branch of the Zizzi restaurant chain and The Mill pub in Salisbury, where witnesses say Skripal began to behave in an agitated manner following lunch with his daughter.

The pair were caught on CCTV walking down an alley near where they fell unconscious on a bench at 4.08pm last Sunday, minutes before emergency services were called.

A passing doctor described Yulia "vomiting and fitting" and losing control of her bodily functions. The doctor moved the 33-year-old into the recovery position, as others tended to her father.

All the symptoms are consistent with nerve agents, which vary in strength according to type, concentration and how they are administered.

"You can impregnate them in liquid form into clothing and fabrics," said an expert. "It looks like they were both exposed at exactly the same time, otherwise one of them would have been able to call the emergency services."

If Sergei Skripal was in hiding, he was hiding in plain sight. The house in Salisbury he moved into a year after being exchanged in a spy swap was bought for cash and registered in the double agent’s own name. He made little attempt to follow a key rule of espionage — be unpredictable and avoid routine.

Instead, the 66-year-old former colonel was a creature of habit: every Sunday he went to the city’s Railway Social Club to have a roast, a few pints and a shot of neat vodka. His local shopkeeper recalled his weekly routine of buying UK£30 of scratch cards.

Born in June 1953 in the Soviet province of Kaliningrad, between Poland and Lithuania, Skripal became fascinated by the West, tuning into the BBC World Service. He married Liudmila in 1972, having their son, Alexander, two years later and Yulia in 1984.

His fearsome reputation as a soldier was forged in the Soviet airborne troop known as Desantniki, and in 1979 he joined the USSR’s invasion of Afghanistan. After graduating from the Diplomatic Military Academy he was recruited to the main intelligence unit, known as GRU, where he specialised in spying in Europe, often by serving as a diplomat.

It was while based in Spain in the Nineties that the Spanish intelligence services realised he was ripe for recruitment as a double agent. However, MI6 handled him, in part because his English was so good.

In 1995 he was given the code name ‘Forthwith’. MI6 bought a timeshare apartment near Malaga to meet him. He was paid up to UK£4,000 each trip and revealed secrets about Russian intelligence officers in Europe.

Around the turn of the Millennium he quit the GRU, but continued to meet colleagues. In 2004, Skripal was filmed being arrested and convicted of “high treason in the form of espionage”. His 13-year sentence to hard labour in Mordovia was cut short in 2010 when he was released in the spy swap.

In 2011, he moved into Salisbury, inviting neighbours to his house-warming.